Effect of the tetrapeptide RGDS on somatic embryogenesis in Daucus carota

The present work examines the requirement for RGD-binding sites, such as th ose in the plasma membrane protein integrin during somatic embryogenesis in carrot (Daucus carota L. cv. Scarlet nantes). It is possible to assay for this requirement by competitively inhibiting binding of the site to the nat ural RGD-containing ligand by adding an excess of synthetic RGDS. We found that treatment of carrot callus cultures with RGDS (2.7 mM) inhibited the d evelopment of a normal shoot pole in carrot somatic embryos. The structures that formed contained separate zones of highly cytoplasmic and vacuolate c ells and no evidence of embryonic organization, although occasionally a roo t-like structure was observed. If the aspartic acid residue in the peptide was replaced by a chemically similar amino acid (glutamic acid), the result ant somatic embryos were indistinguishable from those developing in untreat ed cultures. These effects are similar to those reported in animal systems where the protein receptor involved has been identified as integrin. Our re sults are thus consistent with the conclusion that a binding site for a mot if similar to the integrin-binding site in a variety of animal extracellula r matrix proteins exists in plants and appears to be important in somatic e mbryo development in carrot.